1. Field of the Invention
The present invention relates to a method used in a communication system and related communication device, and more particularly, to a method of handling a damage information-assisted hybrid ARQ scheme in a communication system and related communication device.
2. Description of the Prior Art
An automatic repeat request (ARQ) scheme is used between a transmitter and a receiver in a communication system to provide reliable data transmission. For example, when the ARQ scheme is used, a packet is transmitted with redundant bits generated by an error detection code (EDC) (e.g. a cyclic redundancy check (CRC) code) by the transmitter to the receiver. After the receiver receives the packet with the redundant bits, the receiver checks correctness of the packet by using the EDC. If the packet is received without any error, the receiver feedbacks an acknowledgment (ACK) to inform the transmitter that the packet has been received correctly. After the transmitter receives the ACK, the transmitter can continue to transmit successive packets. Oppositely, if the packet is received with an error, the receiver feedbacks a negative acknowledgment (NACK) to inform the transmitter that the packet has not been received correctly. After the transmitter receives the NACK, the transmitter retransmits the packet until an ACK is received.
However, the reliable data transmission using the ACK scheme is inefficient. For example, there is a great opportunity that the packet is received with small errors (e.g. a small number of error bits) and it is not necessary to retransmit the whole packet such that resources of time, frequency and/or code are wasted. In this situation, if an error correction code (ECC) (e.g. a convolutional code) is used, the packet can easily be corrected and the resources are saved. Therefore, a number of feeding back the NACK and retransmission can be reduced. A hybrid automatic repeat request (HARQ) scheme is proposed based on this concept.
The HARQ scheme is used in the communication system to provide both efficient and reliable data transmissions. Different from the ARQ scheme, both the EDC and the ECC are used in the HARQ scheme. For example, when the HARQ scheme is used, the transmitter encodes the packet into a payload by using the EDC and the ECC, and transmits the payload to the receiver. After the receiver receives the payload, the receiver decodes the payload into a decoded packet by using the ECC and checks the decoded packet by using the EDC. If the decoded packet is correct without any error, the receiver feedbacks an ACK to inform the transmitter that the packet has been received correctly. Oppositely, if there is an error with the decoded packet, the receiver feedbacks the NACK to the transmitter. Since the packet with the small errors can be correctly decoded by using the ECC without feeding back the NACK, i.e., requesting a retransmission, throughput of the communication system is increased due to fewer retransmissions.
Furthermore, there are three types of the HARQ scheme: type-I, type-II and type-III schemes. In the type-I HARQ scheme, a receiver decodes a newly retransmitted payload and discards the payloads of the previous receptions. Therefore, information in the payload of the previous transmission is not exploited, and performance loss is caused. In the type-II and type-III HARQ schemes, the receiver combines the newly retransmitted payload and the payloads of the previous receptions to perform a joint decoding. Therefore, comparing with the type-I HARQ scheme, performances of the type-II and type-III HARQ schemes are improved due to a combination of the payloads. Besides, there are two methods for performing the combination: chase combining (CC) and incremental redundancy (IR). When using the chase combining, the same payload is transmitted in a retransmission. When using the incremental redundancy, a different part of the payload with a fixed size is transmitted in the retransmission. It has been shown that performance of the incremental redundancy is better than performance of the chase combining.
However, the performance of the incremental redundancy is limited due to the fixed size of the retransmitted payload. In detail, an amount of errors in the decoded packet is different in each retransmission. Accordingly, amounts of redundant bits and information of the packet required in the retransmitted payload for the receiver to perform the joint decoding successfully are also different. In this situation, the fixed size of the retransmitted payload may be excessive or insufficient for accommodating the amounts of redundant bits and information of the packet. Not only the resources may be wasted when the fixed size is too large, but more retransmissions may be needed when the fixed size is too small. Therefore, there is a room to improve the performance by adjusting the size of the retransmitted payload in each retransmission. How to adjust the size of the retransmitted payload in each retransmission of the HARQ scheme including the type-II or the type-III such that a better performance is achieved is a topic for discussion.